The objectives of an electrically powered tankless liquid heating device include, at a minimum, provision of the heated liquid on demand, regulation of the temperature of the heated liquid so as not to exceed a maximum temperature set point, operation below a maximum electrical current set-point, safety of operation, minimal disturbance to the power supply and low cost to manufacture. Prior art liquid heating devices have attempted to achieve these objectives, but have been only partially able to do so.
Most prior art electrically powered tankless liquid heating devices use resistance type electrical heating elements to heat the liquid. Although the use of electrical heating elements is well known and widely practiced, in tankless liquid heating devices, they suffer from considerable disadvantages. One of the most important of these is the occurrence of “dry firing”, i.e., operation of the heating element when it is not completely immersed in the liquid, or when excessive deposits are formed along the surface of the heating element, thus enabling operation of the heating element outside of its safe temperature range and introducing the possibility of shortened life span, element failure, system meltdown, or even fire. Additional functional and costly components are required to address this. Maus, in U.S. Pat. No. 4,900,896, provides an example of such a heater. A flow detection switch (which must carry the entire electrical current consumed by the heating elements) detects the condition of no water flow, thus preventing dry firing of the heating elements where there is insufficient water in the heating chamber. However, when the heating element is covered with deposits that are relatively thermally non-conducting, the thermostat is not thermally connected to the heating element and thus the thermostat does nothing to prevent overheating of the electric heating element. Other tankless water heaters using electric heating elements that suffer the same disadvantage and the mechanisms to address it are described in U.S. Pat. Nos. 5,216,743 issued to Seitz, 5,325,822 issued to Fernandez, 5,408,578 issued to Bolivar, 5,479,558 White, Jr. et al, 5,866,880 issued to Seitz et al, 6,080,971 issued to Seitz et al, U.S. Pat. Nos. 6,246,831 issued to Seitz et al, and 6,834,160 issued to Chen-Lung et al. The primary mechanism in '743 is an automatic vapor release outlet to ensure that the temperature sensors sense liquid temperature. This mechanism clearly does not function after the heater has been drained for servicing or for periods of no use. In '822, liquid level sensors are used. However, these are only effective in one mounting orientation of the heater. '578 provides two ports between two heating chambers to ensure that water enters the two chambers more or less equally, thereby preventing that one of the heating elements in one of the chambers can overheat while the other is filling with water. A flow-sensing switch is also used to prevent application of power unless water flow is detected. However, a flow-sensing switch is generally expensive and not reliable. '558 uses the combination of a sophisticated flow detector and thermal sensors, one for regulating temperature, the other for sensing an over temperature condition. The flow detector uses a plunger that is constrained to move vertically, thus constraining the heater to installation in only one orientation. Besides, as described, it is subject to binding and getting stuck in one position, including possibly a position that indicates the existence of water flow when there is none. This solution is expensive, unreliable, and suffers the same problems as '896. '880 provides high temperature limit switches. These are inoperative when there is not a high thermal conductivity thermal path between heaters and the switches, such as when the heater is without water. The '971 and '831 patents provide over temperature switches thereby suffering the previously mentioned disadvantages.
Another disadvantage of liquid heaters that utilize resistance type electric heating elements is that the elements themselves have substantial thermal mass and thermal resistance. This creates the problem of how to manage the latent heat (the heat which has not yet escaped) of the elements when the liquid flow rate is abruptly reduced to near zero or zero. This latent heat must be absorbed by the liquid surrounding the elements. However, doing so increases the temperature of the surrounding liquid, possibly to an undesirable extent. Thus, the volume of the heating chambers must be made larger to avoid overheating of the liquid, for example, to prevent scalding if the liquid heater is a domestic hot water heater. This is also necessary to stabilize the operation of any temperature control loop or else high variations in temperature of the heated liquid will occur. However, these larger heating chambers make it difficult to respond to demand changes, especially when the water flow rate starts from zero.
As previously mentioned, deposits tend to form on the heating elements. Seitz discloses that the amount of mineral deposition is a function of the maximum heating element temperature in '880, and thus the desirablity of providing power to the heating elements as a function of the power needed to heat the water passing through the disclosed heater to minimize such depositions. In the '558 patent, White, Jr. also identifies a different reason for doing this—to minimize power supply voltage fluctuations due to heater power demands that can cause flickering of lights. Unfortunately, the best semiconductor devices for controlling current to electrically powered water heaters are essentially switches (they can be opened and closed, but they don't provide a means for regulating current), thus making this a significant problem. White Jr. addresses this by incorporating multiple equally sized heating elements. However, this only reduces the magnitude of the potential power supply voltage variations by a factor of the number of heating elements, in the case of his example, four. The '880 patent echoes this approach. Seitz, in the '971 and '831 patents, discloses various methods for minimizing the power supply variations caused by variations in the heater power demand and the visible flickering of lights and electrical interference that results there from. These methods generally relate to the use of multiple heat elements and the timing of the application of power to them so as to minimize power supply current fluctuations, or to make these power supply fluctuations such that they are not readily perceived. These lead to a relatively high level of design complexity and a correspondingly high manufacturing cost.
The predominant alternative to using heating elements to heat the liquid is to pass an electrical current through the liquid by passing it between two electrodes between which a voltage exists. The voltage is preferably an AC voltage so as to avoid electrolysis of the liquid. This method is known as direct electrical resistance (DER) heating. Probably the most common application of this approach (although relatively crude) is in vaporizers used to humidify room environments. One reason for the popularity of the approach is that it is intrinsically safe: no electrical current can flow if there is no liquid between the electrodes.
One example of a DER liquid heater is disclosed in U.S. Pat. No. 6,130,990 issued to Herrick et al for use in a beverage dispenser. The advantages of “rapid and efficient transfer of electrical energy into the water as thermal energy while reducing the energy loss associated with indirect heating methods” are disclosed. One of the disadvantages of the DER method, however, is that the amount of electrical current drawn by the liquid between the electrodes, and therefore the amount of heat delivered to the liquid, is determined by the electrical conductivity of the liquid, a parameter that can vary quite widely, for example 10 to 1. One method of controlling the temperature contemplated in this patent is by varying the water flow rate. Another is by varying the electrical power delivered to the water, which would require varying the power supply voltage. A third involves mechanically adjusting the distance between the electrodes. It is evident that accommodating such wide range of liquid conductivities by any of these methods is quite difficult. In fact, the inventors contemplate the possibility of treating the water with minerals prior to passing it through the heater in order to increase the water conductivity. In U.S. Pat. No. 6,522,834 also issued to Herrick et al, which is a continuation in part of the '990 patent, a new element, a power supplier, is introduced specifically to overcome this issue. Essentially, it is a power converter that receives power from a convention power supply (for example, 220VAC @ 60 Hz), and converts it such that the output voltage is adjustable and which may have a frequency range from 50 Hz to 200 KHz. This was apparently driven by the need to accommodate the large range of water conductivities and the inadequacy of the other previously mentioned methods. U.S. Pat. No. 6,640,048 issued Novotny et al discloses a DER liquid heater that provides another adjustment mechanism that addresses the wide range of liquid conductivities. It mechanically varies the area of the electrodes (and the effective distance between them) by adjustably interposing an electrically non-conducting current gating plate between the electrodes, thus adjusting the electrical conductance of the heating zone comprising the electrodes and the liquid between them. However, no disclosure of the range of adjustability of the device is disclosed. Furthermore, the mechanical adjustment involves the translation of motion across a liquid to air barrier, something that is difficult to achieve reliably and at low cost.
DER liquid heaters must also address other difficulties that are in common with heaters utilizing resistance type electrical heating elements. An example of these is the use of a flow switch to control the application of power to the heater. Flow switches are generally characterized by a flow rate threshold, below which they do not indicate a flow, although a low flow may be present. This allows for unheated liquid to leave the heater at low flow rates (unlike conventional tank type heaters), and it tends to generate a delay between the time liquid flow is demanded and the time fully heated liquid is finally delivered thus creating a wastage of liquid. This, together with the presence of orientation limitations, unreliable functioning and cost must be overcome in a tankless liquid heating device that meets the objectives cited above. Additionally, the previously mentioned difficulties associated with latent heat management, the design and operation of temperature control loops, formation of deposits, and minimization of power supply variations and the corresponding light flicker must be overcome.